Ensure that we consistently check contains: paint for fixed position containment.

third_party/WebKit/Source/web/tests/data/rgm_contains_fixed_position_test.html

Index: third_party/WebKit/Source/web/tests/data/rgm_contains_fixed_position_test.html
diff --git a/third_party/WebKit/Source/web/tests/data/rgm_contains_fixed_position_test.html b/third_party/WebKit/Source/web/tests/data/rgm_contains_fixed_position_test.html
new file mode 100644
index 0000000000000000000000000000000000000000..200ffedae1a530addaa35ab6e141a59a7262cb22
--- /dev/null
+++ b/third_party/WebKit/Source/web/tests/data/rgm_contains_fixed_position_test.html
@@ -0,0 +1,32 @@
+<style>
+    div {
+        width: 100px;
+        height: 100px;
+    }
+    .container {
+        position: absolute;
+        top: 100px;
+    }
+    #contains-paint {
+        contain: paint;
+    }
+    #has-transform {
+        will-change: transform;
+    }
+    .fixed {
+        position: fixed;
+        top: 0;
+    }
+    .spacer {
+        height: 2000px;
+    }
+</style>
+<script>
+onload = function() {
+    window.scrollTo(0, 50);
+};
+</script>
+<div class="container" id="simple-container"><div id="fixed1" class="fixed"></div></div>
+<div class="container" id="has-transform"><div id="fixed2" class="fixed"></div></div>
+<div class="container" id="contains-paint"><div id="fixed3" class="fixed"></div></div>
+<div class="spacer"></div>